The case of Twin B involves the decision to send a newborn to a less intensive Level 2 special care nursery (SCN) than to the Level 3 neonatal intensive care unit (NICU) that is considered optimal by the physician. The physician’s acceptance of the transfer is against the child’s best interest and is due to parental convenience. In analyzing the case, we reject the best interest standard. Our rejection is partly supported by the views of Douglas Diekema, John Hardwig, and Lannie Ross. Instead of the best interest standard, we offer and defend an approach we base on a microeconomic analysis of externalities, such as those involved with automobile emissions. This extends our previously presented general microeconomic approach to patient decision-making. It provides a clearer way to evaluate situations, like those of Twin B, in which burdens faced by family members may be used to determine the appropriate level of treatment for a decisionally incapable patient